Tuesday, July 17, 2012

Various types of air conditioning systems (also
called types ofHVAC systems) have been designed to cater to the demands of HVAC in
various applications in the best economical and energy efficient way.

In institutional,
commercial, and residential buildings, air-conditioning systems are mainly for
the Occupants’ health and comfort. They are often called comfortair conditioning systems.

In manufacturing buildings,
air conditioning systems are provided for product processing, or for the health
and comfort of workers as well as processing, and are called processingair conditioning systems.

Based on their size,
construction, and operating characteristics, air-conditioning systems can be classified
as the following.

Individual Air conditioning System(Window or split ac)

An
individual room or individual air conditioning system normally employs either a
single, self-contained, packaged room air conditioner (installed in a window or
through a wall) or separate indoor and outdoor units to serve an individual
room (split air conditioner)you can see a split air conditioner in the figure shown above.“Self contained, packaged” means factory assembled
in one package and ready for use.

Space Air conditioning System

These
air conditioning systems have their air-conditioning — cooling, heating and
filtration—performed predominantly in or above the conditioned space. Outdoor
air is supplied by a separate outdoor ventilation system (as a part of HVAC system).

Packaged Air conditioning System

These air conditioning
systems are installed with a single self
contained, factory-assembled packaged unit (PU) or two split units: an indoor
air handler, normally with duct work, and an outdoor condensing unit with
refrigeration compressor(s) and condenser. In a packaged system, air is cooled
mainly by direct expansion of refrigerant in coils called DX coils and heated
by gas furnace, electric heating, or a heat pump effect, which is the reverse
of a refrigeration cycle.

Central Air conditioning System

A central air
conditioning system in hvac system uses chilled water or hot water from a central plant to
cool and heat the air at the cooling or heating coils in an air handling unit
(AHU). For energy transport, the heat capacity of water is about 3400 times
greater than that of air. Central systems are built-up systems assembled and
installed on the site.

Capacity of absorption chillers is controlled by varying the
input thermal energy to generator.While input thermal energy to absorption
chillers is regulated with reference to load on chillers .Suppose load on
absorption chillers changes then the outlet temperature of chilled water will also be changed little and this is generally sensed by chiller instrumentation.Chiller
controller now regulates the valve of thermal energy with respect to the change
in thermal load.

This closing of valve will reduce the heat input to generator of
absorption chillers and temperature in generator will be reduced. By this
technique of capacity modulation,heat input to chiller is lowered in generator so accordingly less vapors of
refrigerant (water in lithium bromide chillers) will be produced .These vapor
get condensed in condenser of chiller and finally reach the evaporator of
chiller at lower pressure.Now because less vapors reach evaporator so less heat
will be extracted from chilled water and refrigeration effect will be reduced
and chilled water temperature will increase accordingly.

Mass flow rate of refrigerant is directly proportional to
load and refrigeration effect.

Generally between 30 and 100 percent of the hvac system design refrigeration load, the burner fired rate is modulated. Below 30
percent of design hvac load, the burner is cycled on and off, and all refrigerant
and solution pumps remain.

On the other hand ,concentration of solution reaching the
absorber from generator will drop accordingly.In other words as less thermal
energy is supplied to generator of chillers, solution reaching the absorber
will be less strong as it would still contain some refrigerant. This little less strong solution
will have less capacity to absorb refrigerant vapors as compared to strong
solution.So accordingly more vapors will remain in evaporator and would
increase the evaporating temperature and pressure in evaporator of absorption chiller. Finally a new balance will be established between gas inlet valve and
load to achieve the preset temperature at outlet of evaporator.

Capacity Modulation of Absorption Chiller as load Increases

Similarly if load on hvacabsorption chiller increases then chilled water temperature
will also be increased accordingly.Chiller need to be adjusted in accordance
with this new load value.Gas valve or steam inlet valve will be opened more to
adjust the refrigeration effect and load on chiller.Rest of effect will be inverse of load decrease.

Wednesday, July 4, 2012

Double effect
absorption chiller is an improvement in single effect lithium bromide chiller (read here all about Single effect absorption chiller ) . Double
effect water chillers have more coefficient of performance and enhanced utilization
of input energy.Basic working principle of these dual effect absorption chillers
remains same as single effect but with addition of some components to vapor
absorption cycle. Absorption cooling is enhanced using the system energy which
would have been otherwise wasted as in case of single effect industrial chiller.In order
to produce chilled water in these double effect chillers,water remains as a
refrigerant and lithium bromide as absorbent in these HVACwater chillers. As
compared to single effect absorption chillers,double effect vapor absorption chiller has an
additional heat exchanger and generator/concentrator.

HVAC double effect absorption chiller

Basics of Double effect Absorption Chiller Operation

HP Generator Process
in Double effect Absorption Chiller Operation

In double effect
absorption chiller,both in direct and indirect fired chiller operation, weak
solution from the absorber is pumped into the HP generator through LT,
condensate and HT heat exchangers. It is heated by operating steam and
concentrated to the intermediate solution, and high temperature refrigerant
vapor is produced.Instead of steam ,hot exhaust gases or waste heat may also be used as in case of direct fired absorption chillers.

In next step of chiller operation,Intermediate
solution enters LP generator through HT heat exchanger in exchanging heat with
weak solution which is passed through the tubes. Intermediate solution in LP generator is heated by the refrigerant
vapor from HP generator, concentrated to the strong solution, releasing
refrigerant vapor at the same time.

Refrigerant vapor
from HP generator is condensed in LP generator to form condensate, which enters
condenser through throttle. Refrigerant vapor formed in the LP generator flows
to the condenser to form condensate also.

At normal
temperature and pressure water boils at 212oF but as pressure is
lowered sufficiently below atmospheric pressure then boiling temperature is
also decreased .Same thing is used in evaporator of absorption chillers(single
and double effect ).These two parts of refrigerant condensate flows into the flash
chamber through U pipe. A part of refrigerant water is flashed to form vapor,
which flows into the re-absorption chamber at the bottom of absorber, while
another part of refrigerant water is cooled, and enters evaporator refrigerant
pan. Refrigerant from evaporator refrigerant pan is pumped over the evaporator
tubes for the refrigeration effect, and evaporates to form vapor by absorbing
heat of chilled water flowing through tubes.

Produced refrigerant
vapor enters absorber, and absorbed by strong solution in the absorber. Chilled
water is cooled and return to the HVAC system of customer. Strong solution is
diluted by absorbing refrigerant vapor in absorber and absorbing flashed
refrigerant vapor in the re-absorption chamber.

Solution Pump in
Double effect Absorption Chiller Operation

Weak solution is then transferred
by solution pump to HP and LP generator for concentration. Heat generated is
carried to atmosphere by cooling water. This process is continued, and
refrigeration effect is repeated.